A hypodermic syringe is described in my U.S. Pat. No. 4,874,381 which has a generally cylindrical syringe body centered on a longitudinal axis and formed with a front end having a needle-receiving fitting, a rear end opposite the front end, and a bypass intermediate the ends. A rear piston is slidable in the body proximal to the rear end and a front piston is slidable in the body between the rear piston and the front end. The pistons define in the syringe body a rear liquid compartment adapted to communicate with a front compartment between the front piston and the front end through the bypass upon displacement of the front piston so that opposite sides thereof are bridged by the bypass. A piston-actuating stem axially engaged by the rear piston extends out of the rear end and is axially displaceable to drive the pistons toward the front end of the body.
Similar such arrangements are describes in U.S. Pat. Nos. 2,591,046, 4,226,236, and 4,613,326. They allow the fluid in the rear compartment to be kept separate from a substance in the front compartment until they are needed. At that time the plunger is pushed longitudinally forward until the front partition piston aligns with the bypass and the fluid in the rear compartment is pumped past the now stationary front piston into the front compartment.
In my earlier patent damping is provided at the rear end of the body for restricting the velocity of the rear piston substantially until it reaches the bypass so as to limit the speed with which liquid from the liquid compartment enters the front compartment. The damping then disengages to permit faster axial displacement of the pistons by the stem. This damping is effected by a cap fitted onto the rear end and formed with a central bore having an internal screwthread. The stem has over part of its length an axially extending shank having an external screwthread threadedly engaging the internal screwthread and the shank has a length such that the external screwthread passes out of the internal screwthread upon rotation of the stem to advance the rear piston as the rear piston reaches the bypass. Thus for the first part of its stroke as the stem is pushed axially forward it must be slowly screwed through the cap on the syringe. Once the screwthread on the front portion of the stem disengages from the screwthread on the cap, which takes place roughly when the rear piston comes to rest against the rear face of the front piston, the screwthreads no longer limit forward advance of the stem for rapid ejection of the mixture in the front compartment.
Such an arrangement is excellent for easily mixed substances. When, however, the substance in the front compartment is, for example, a powder that does not readily go into suspension or solution with the liquid in the rear compartment, the above-described system provides no ready procedure for mixing. The user must shake the now ready syringe until the substance is fully mixed. If the mixture is incomplete it is possible for undissolved particles to block the needle or to remain in the syringe.